The present invention relates to transmission channel equalizers and in particular to an equalizer system suitable for use with a variety of modulated signals such as quadrature amplitude modulated (QAM) signals, vestigial side-band modulated (VSB) signals or NTSC video signals.
Digital data communication systems generally transmit symbols from a finite alphabet, A, at discrete, (usually periodic) time instants known as baud instances. These symbols can be used to modulate a Radio Frequency (RF) carrier""s amplitude and phase for transmission over a variety of media (e.g. terrestrial, underwater, cable, etc.) to a remote receiver or user. There exist various modulation formats, which may be tailored to the application or transmission medium. Quadrature Amplitude Modulation (QAM) however, appears to be the de facto standard for cable TV, and Vestigial Sideband Amplitude Modulation (VSB) has been mandated by the Federal Communications Commission (FCC) as the format for broadcasting Digital Television signals with deployment beginning in November, 1998. Details of the VSB modulation system may be found in Annex D of xe2x80x9cATSC Digital Television Standardxe2x80x9d Advanced Television Systems Committee, Document A/53, September, 1995, which is incorporated herein by reference for its teaching on VSB modulation and demodulation of digital television signals.
To provide reliable data estimates from either of these signaling formats (QAM or VSB), a receiver desirably performs a number of functions, including (but not limited to) RF demodulation, synchronization of a carrier loop to the RF carrier, synchronization of the clock signal to the baud sampling instants, equalization, and decoding. Because the transmitted signal is subject to a propagation medium which has a frequency response characteristic and may introduce distortion, an equalizer is used to compensate for the frequency response characteristic of the transmission channel and to mitigate the distortion caused by the transmission channel. The transmitted signal may also be distorted in the transmitter or the receiver, for example, by non-linear components, poorly terminated transmission lines, or finite processing hardware implementation. Collectively, these types of distortion are referred to herein as channel impairments. In a typical receiver, an equalizer may be used to correct the channel impaired signal by mitigating the effects of channel impairment distortion.
The characteristics of the distortion, however, are generally unknown to the receiver. Accordingly many equalizers use adaptive methods (i) for acquisition, to adjust the equalizer parameters from a cold-start initialization to a setting that removes the channel distortion, and (ii) for tracking, to allow the equalizer parameters to follow variations in the channel over time. There exist various equalizer architectures and many methods to adapt the equalizer parameters. One such equalizer is described in a paper by David A. Bryan entitled xe2x80x9cQAM for Terrestrial and Cable Transmissionxe2x80x9d IEEE Transactions On Consumer Electronics vol. 41, no. 3, pp. 383-391, August, 1995 which is incorporated herein by reference for its teachings on QAM equalization. This equalization system employs a Constant Modulus Algorithm (CMA) to set the equalizer tap coefficients to first acquire the signal and then switches to a Decision Directed (DD) coefficient update algorithm to dynamically track changes in the received signal introduced by the transmission channel.
Another exemplary equalization system is a ghost cancellation system for broadcast television signals which conform to the standards adopted by the National Television Standards Committee (NTSC). This exemplary system uses an infinite-impulse response (IIR) filter, a training signal and a Least Square (LS) update algorithm to establish optimum coefficients for the IIR filter. The system is described in U.S. Pat. No. 4,864,403 to Tzy-Hong Chao et al. entitled ADAPTIVE TELEVISION GHOST CANCELLATION SYSTEM INCLUDING FILTER CIRCUITRY WITH NON-INTEGER SAMPLE DELAY, which is incorporated by reference herein for its teachings on channel equalization techniques.
The present invention is embodied in a transmission channel equalizer system which may be used to process modulated radio frequency signals or acoustic signals. The processed signals may be analog signals or signals that have been modulated to convey digital symbols. The equalizer system includes a sparse digital filter having coefficients which are adaptively updated.
According to one aspect of the invention, the coefficients are adaptively updated without reference to a training signal.
According to another aspect of the invention, the selection of the sparse taps and the association of coefficients with the taps is also adaptive.
According to yet another aspect of the invention, the filter system includes a finite impulse response (FIR) filter which processes modulated pass-band RF signals and an infinite impulse response (IIR) filter which also processes pass-band signals. At least one of the FIR and IIR filters is implemented as a sparse filter.
According to yet another aspect of the invention, the IIR filter processes demodulated base-band signals.
According to yet another aspect of the invention, the filter system may process vestigial side-band modulated (VSB) signals;
According to yet another aspect of the invention, the filter system may process quadrature amplitude modulated (QAM) signals;
According to yet another aspect of the invention, the filter system may be dynamically changed to process either QAM or VSB signals.
According to yet another aspect of the invention, the filter system uses a single axis constant modulus algorithm (SACMA) to update the filter coefficients during acquisition of a VSB signal.
According to yet another aspect of the invention, the filter system includes apparatus which converts a VSB signal into a QAM signal for processing by the carrier recovery circuitry, thus allowing circuitry reuse with QAM and VSB signals.